This invention relates to an RF excitation and/or receiver for use in an NMR apparatus, the probe comprising a generally tubular member acting as an RF resonator and having a plurality of circumferentially spaced axial conductors extending between a pair of spaced ring-like conductors, and a plurality of capacitive elements spaced along and interrupting the ring-like conductors. In particular the invention concerns a device for irradiating a sample with radio frequency (RF) energy and receiving NMR signals from it. The device may be used as either a transmitter or receiver, or both.
A device of this kind is, for example, known in the art from EP 0 177 855 B1.
The principal governing relationship in NMR is the Larmor equation: EQU .omega.=.gamma.B.sub.0
where .omega. is the Larmor precessional frequency, .gamma. is the nuclei specific gyromagnetic ratio and B.sub.0 is the applied magnetic field. This equation applies to the situation where an ensemble of nuclei possessing nuclear spin are subjected to a strong magnetic field. A number of possible energy levels are developed by the interaction of the nuclear spins (which possess magnetic moments) and the applied field. In order to induce transitions between these energy levels, RF energy (B.sub.1 field) is applied to the ensemble at the Larmor precessional frequency, with a B.sub.1 direction orthogonal to the direction of the applied field.
After the RF excitation is removed or ceases, the spin ensemble tends to return to its original state and in doing so emits energy. This is the received NMR signal. This signal can be detected by the same device (termed an RF probe) that was used to transmit the RF excitation, or by a separate probe. The (or each) probe normally comprises a coil or coil-like structure. In either case the probe(s) is/are tuned to, or near to the Larmor frequency.
It is most important in NMR and magnetic resonance imaging (MRI) experiments to maximize the signal-to-noise ratio (SNR) of the experiment, and to irradiate all parts of the sample with the same strength RF field. Similarly, it is important that the NMR signal from all parts in the sample be received by the RF probe with the correct weighting. Perhaps the two most important characteristics of an RF probe are the provision of a homogeneous B.sub.1 field in the volume of the probe coil, and the possession of a high quality factor (Q). By reciprocity, if a coil provides homogeneous excitation it will also receive NMR signals in a homogeneous fashion. In this specification it will be assumed that discussions of excitation distributions of coils apply with equal relevance to their use as NMR receivers.
The Q of a coil is defined as 2.pi. times the ratio of the time-averaged stored energy in the cavity to the energy loss per cycle. The Q of a coil has a profound effect on the SNR of the NMR experience (SNR .alpha. (Q).sup.1/2).
Prior art probes have been designed to provide a homogeneous B.sub.1 field without regard to Q, or a high Q coil without regard to RF field homogeneity, since optimization of one is usually at the expense of degradation of the other.
It is an object of this invention to provide an RF coil that provides both a substantially homogeneous RF field and a high quality factor.
It is a preferred object of this invention to provide an RF coil which optimizes both the homogeneity of the RF field and the Q of the probe for a particular situation.